1. Technical Field
A method for forming a bottle shaped trench in a semiconductor substrate is described herein. The term "bottle shaped trench" as used herein refers to a trench having an upper part defined by a positively sloped periphery and a lower part defined by a negatively sloped periphery so as to define a smaller diameter neck portion between the upper and lower parts of the trench. More particularly, a bottle shaped trench is formed in a semiconductor substrate using a single etch process by varying the etch conditions.
2. Background of Related Art
Methods for etching a deep trench to provide storage capacitance in a semiconductor device are known. An example of such a method is reactive ion etching. Generally, reactive ion etching is a method which first creates a plasma gas composition in a reaction chamber by a radio frequency (RF) energy field. The plasma gas composition will usually contain neutrals, radicals and ions. The latter can be accelerated by the electric field toward the surface of the semiconductor substrate. Radicals reach the semiconductor surface by diffusion. Upon colliding with the surface of the semiconductor device, the accelerated ions together with the radicals remove material from the exposed surface of the semiconductor device. The ions together with the radicals react with atoms or molecules in the material to be etched to produce intermediate products or volatile by-products which can be removed from the reaction chamber. A sidewall passivating film normally forms from the reaction between the intermediate products or volatile by-products with species from the mask material or the process gases. This film tends to inhibit etching, shrink the physical size of the mask opening and thereby affect trench shape.
To further increase the storage capacitance in a semiconductor device, a bottle shaped trench has been developed. Ozaki et al., 0.228 .mu.m.sup.2 Trench Cell Technologies with Bottle-Shaped Capacitor for 1 Gbit DRAMs, Technical Digest; Vol. 27.3.1, p. 661 (1995), discloses a multi-step method for forming a bottle shaped trench. The Ozaki et al. method forms a bottle shaped trench by (1) forming a trench by conventional DT silicon reactive ion etching; (2) forming a collar oxide at the upper portion of the trench by selective oxidation; (3) depositing into the trench in-situ phosphorous doped polysilicon; (4) annealing phosphorous dope into the trench sidewall at the bottom portion of the trench; and (5) removing the polysilicon by chemical dry etching to enlarge the diameter of the bottom portion of the trench.
It would be desirable to provide a more expedient and simplified method for forming a bottle shaped trench in a semiconductor device to increase the storage capacitance thereof. The method of this invention allows to form bottle shaped trenches during the deep trench etch process without any additional processing steps.